Thermoelectric conversion elements are devices that are expected to see practical use because of their efficient utilization of the high levels of thermal energy required in recent industrial fields. An extremely broad range of applications have been investigated, such as a system for converting waste heat into electrical energy, small, portable electric generators for easily obtaining electricity outdoors, flame sensors for gas equipment, and so forth.
This conversion efficiency from thermal energy to electrical energy is a function of the Figure of merit ZT, and rises in proportion to ZT. This Figure of merit ZT is expressed by Formula 1.ZT=α2σT/˜k  Formula 1
Here, α is the Seebeck coefficient of the thermoelectric material, σ is the electrical conductivity, κ is the thermal conductivity, and T is the absolute temperature expressed as the average value for the thermoelectric element on the high temperature side (TH) and the low temperature side (TL).
Silicides such as FeSi2 and SiGe, which are thermoelectric conversion materials that have been known up to now, are abundant natural resources, but the former has a Figure of merit ZT of 0.2 or less, its conversion efficiency is low, and its usable temperature range is extremely narrow, while no decrease in thermal conductivity is seen with the latter unless the germanium content is about 20 to 30 at %, and germanium is a scarce resource. Also, silicon and germanium have a state in which there is a broad liquidus and solidus for complete solid solution, and it is difficult to produce a uniform composition with melting or ZL (zone leveling), to name just two of the problems which have impeded industrial application. For these reasons, the above-mentioned materials have not found widespread use.
The thermoelectric materials with the highest Figure of merit at the present time are IrSb3 having a skutterudite-type crystal structure, and BiTe, PbTe, and other such chalcogen compounds, which are known to provide highly efficient thermoelectric conversion capability, but from the standpoint of protecting the global environment, the use of these heavy metal elements is expected to be restricted in the future.
Silicon, meanwhile, has a high Seebeck coefficient, but has extremely high thermal conductivity, and is therefore not considered suitable as a high efficiency thermoelectric material, and research into the thermoelectric characteristics thereof has been limited to silicon with a carrier concentration of 1018 (M/m3) or less.